The present invention relates to a magnetic disk drive, and more particularly to a magnetic disk drive having a mechanism of controlling a sense current of a magnetic head.
Magnetic disk drives are indispensable in today's advanced information society as devices capable of recording a large amount of data, and are accordingly required to stably read/write data. A magnetic disk drive comprises: a magnetic recording medium (magnetic disk) to which data is written; a magnetic head for reading/writing data; a voice coil motor for moving the magnetic head; and other circuits. The magnetic head includes a write head for writing data, and a read head for reading data that is written to the magnetic disk. As an indicator for indicating read/write properties, there is an error rate. The error rate means a degree to which errors are included in read data, and shows that if a value of the error rate is small, the number of errors is small, which achieves good read/write properties. In general, in the SMART (Self Monitoring Analysis and Reporting Technology) function, which is a technology for improving the reliability of magnetic disk drives, there is a function of measuring an error rate, which makes it possible to measure an error rate in a magnetic disk drive.
Heretofore, magnetic disk drives are usually used for PC-related purposes, or used for large-scale storage systems such as RAID. However, their new uses are spreading these days. For example, car navigation systems, and portable terminals (cellular phones, and music terminals), are increasingly equipped with a magnetic disk drive taking advantage of the large storage capacity as well as the fast access speed. Reflecting the widespread uses, magnetic disk drives are required to operate in a temperature range that is wider than the currently used temperature range.
What is problematic here is in particular the performance of a magnetic disk drive at cold temperature. The coercivity of a magnetic disk increases as the environmental temperature decreases. Accordingly, writing becomes relatively difficult under the same write conditions, resulting in the degradation in read/write properties. With the object of preventing the degradation from occurring, as disclosed in patent document 1 (Japanese Patent Laid-Open No. 2004-5852), for example, what is proposed is the idea of aligning an overshoot electric current at the time of writing to optimize write conditions, and thereby improving read/write properties. However, with the increase in recording density of magnetic disk drives, the track width of a write head and that of a read head decrease. Therefore, if a decrease in output and the difficulty in positioning are taken into consideration, it is not easy to ensure the properties only by improving a write system.
In a magnetic disk drive, a sense current to flow through a read head is set at such a value that a desired element life is ensured at high temperature (for example, 70° C.). The element life is strongly dependent on an increase in temperature of an element. Accordingly, if the environmental temperature is low, a larger increase in temperature of the element is allowed, which makes it possible to further increase the sense current. From this point of view, what is considered as an effective method is to control the sense current so as to suppress the degradation in properties caused by a change in environmental temperature. As disclosed ideas relating to this, there are methods as follows: for example, as disclosed in patent document 2 (Japanese Patent Laid-Open No. Hei 11-195211), changing a sense current on the basis of a table predetermined in response to the temperature; as disclosed in patent document 3 (Japanese Patent Laid-Open No. 2001-23106), using the above-mentioned table, or controlling a sense current as a predetermined function of the resistance of an element and the temperature; or the like.